In digital communication, usually, the error correction which corrects an error in data generated in a transmission line is often used. The error correction is realized by error correction encoding on a transmission side and error correction decoding on a reception side.
There are several types of error correction encoding, and in recent years, as codes having a high error correction capability, the Turbo code and the LDPC code are known. These codes provide a high error correction effect respectively by the Turbo decoding and the LDPC decoding on the reception side.
Information input to those decoders is generally referred to as “soft decision value”. Digital data has bit information, which is “0” or “1”, and therefore, there are a method of inputting information determined as a binary value (referred to as “hard decision value”), which is 0 or 1, to a decoder, and a method of inputting, instead of the binary value, a soft decision value represented as a probability, a likelihood, or a log likelihood of being 0 or 1 to the decoder. In particular, the latter method is referred to as soft decision decoding, is high in compatibility with the Turbo decoding and the LDPC decoding, and provides a high error correction effect compared with the former method (referred to as hard decision decoding).
A circuit for generating a soft decision value provided for the decoder is referred to as a soft decision value generation circuit. The soft decision value generation circuit generates a soft decision value from a received signal (the signal is hereinafter referred to as symbol) depending on a used digital modulation scheme. In digital communication assuming coherent detection, generally digital modulation schemes such as the phase shift keying (PSK) and the quadrature amplitude modulation (QAM), which are excellent in transmission characteristic, are used. Generation methods for the soft decision value in the PSK and the QAM are disclosed in, for example, Non-Patent Literature 1.
On the other hand, in the binary PSK (BPSK) or the quadrature PSK (QPSK), there are modulation methods such as differentially encoded (DE) BPSK and DEQPSK of differentially encoding modulation symbols. The differentially encoded modulation symbols can be decoded (soft decision value can be generated) by carrying out delay detection in the receiver, and usually eliminate necessity of the coherent detection. It should be noted, however, that by carrying out the coherent detection in the receiver, a higher reception capability than that obtained when the delay detection is carried out can be provided (for example, Non-Patent Literature 2 reports the reception capability).
In Patent Literature 1, in the modulation scheme which employs differential encoding, such as the DBPSK and the DEQPSK, an invention involving respectively obtaining, for a received symbol, the minimum value in error from a pattern (modulation symbol candidate) causing a transmitted bit to be 1 and the minimum value in error from a modulation symbol candidate causing a transmitted bit to be 0, and setting a difference thereof as reliability information, namely as a soft decision value is disclosed.